Pip-tree

- Adds `mypy` type checking and fixes types
- Fixes a potential bug when regex pattern matching on package names would result in `None`

- Small bug fix to change the types of functions available in the `__all__` variable to strings

- Refactored code
- Unwrapped functions from `PipTree` class, exposed them outside the package
- Removed an entire loop and function, now Pip Tree will run even faster
- Broke out `cli` logic to a separate module
- Use smarter enumeration for package count
- Added type hinting
- Updated documentation

- Drops support for Python 3.6
- Removes unused `mock` library

- Switched from shelling out to Pip to using the internal Pip API natively via Python (closes 4 and closes 2), this change makes the previous ~1 minute operation now take ~1 second!
- Adds `updated` field indicating when the package was installed or updated (closes 5)
- The `requires` and `required-by` keys are now lists instead of comma separated strings, they also include the version the requirements are pinned to
- Using `argparse` instead of environment variable to specify path to site-packages
- Separated out code better into classes and additional functions
- 100% code coverage
- Converted classmethods to staticmethods

- Removes pip version warnings from output
- Fixes `local variable 'i' referenced before assignment` error
- Matching pip command timeout by setting the subprocess timeout to 15 from 10 seconds
- Running pip commands in isolated mode which ignores custom configuration
- Running pip commands while skipping user input (if ever applicable)
- Small speed improvements were made by shifting logic around